A. Technical Field
The present invention relates to semiconductor devices, and more particularly, to high electron mobility transistors.
B. Background of the Invention
The high electron mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a type of field effect transistor (FET) in which a hetero-junction between a channel layer and a barrier layer whose electron affinity is smaller than that of the channel layer is formed. HEMT transistors are able to operate at higher frequencies than ordinary transistors, up to millimeter wave frequencies, and are typically used in high-frequency and high-power products, such as power amplifier between mobile phone stations and phased array laser in military applications.
In general, HEMTs for operation at the radio frequency (RF) range require higher breakdown voltage than the ordinary transistors, where the breakdown voltage is the maximum voltage that the gate of a transistor can handle. In existing HEMTs, a source-connected gate field plate has been used to increase the breakdown voltage. However, with the advent of modern mobile communication technology, a demand for HEMTs with higher breakdown voltages has been continuously increased. Also, in order to get good linearity, a gate-drain capacitance (Cgd) value needs to remain flat in the dynamic driving range.
Also, HEMTs designed to operate at high power ranges may generate high heat energy. Thus, they need to be designed for low output resistances so that they can deliver large currents to the load, and good junction insulation to withstand high voltages. As most heat energy is generated at the heterojunction, the area of this junction can be made as large as possible so that the heat energy can be dissipated very quickly to thereby prevent overheat. However, in many high power applications, the form factor of HEMTs may impose limitations on the size of the device area, resulting in the limitation to the maximum power that the HEMTs can handle.
As such, there is a need for HEMTs that have high breakdown voltages, flat Cgd value in the dynamic driving range, and enhanced heat dissipation mechanisms, to thereby increase the maximum voltage, linearity, and power ratings at various applications, especially at radio frequency ranges.